It is known that a finely machined surface can be obtained in a wire electrical discharge machining when an electrical discharge of short duration is generated repeatedly by applying a high-frequency voltage in a gap (hereinafter, “a machining gap” or “an inter-pole gap”) between a wire electrode and a workpiece.
For example, Japanese Patent Application Laid-open No. S61-260915 (power source for electric discharge machining) discloses that a machined surface less than 1 μmRmax can be obtained if a frequency of the high-frequency voltage that is applied in the machining gap is between 1.0 megahertz and 5.0 megahertz.
Japanese Patent Application Laid-open No. H7-9258 (method and device for electric discharge machining, electrostatic capacitance device and inductance varying device both applicable thereto) disclose that a machined surface less than 5 μmRmax can be obtained if the frequency of the high-frequency voltage applied in the machining gap is between 7.0 megahertz and 30 megahertz.
FIG. 1 is a block diagram of a power supply source that is commonly used in a wire electrical discharge machining apparatus. This power supply source includes a direct current power source 101 and a high-frequency oscillator amplification-circuit 102. The direct current power source 101 supplies power to the high-frequency oscillator amplification-circuit 102. The high-frequency oscillator amplification-circuit 102 generates a high-frequency voltage based on the power received from the direct current power source 101. The high-frequency voltage is applied to a wire electrode 103 and a workpiece 104. The wire electrode 103 and the workpiece 104 are located, at a site where an electric discharge is to be generated, in an opposite manner with a designated gap, i.e., a machining gap 105.
The direct current power source 101 generates a constant direct current voltage or a direct current power in response to an external instruction. The high-frequency oscillator amplification-circuit 102 generates the high-frequency voltage based on the direct current voltage or the direct current power. The high-frequency oscillator amplification-circuit 102 applies a high-frequency voltage in the machining gap 105. As a result, a high-frequency electric discharge is generated in the machining gap 105 and the workpiece is machined by the energy of the electric discharge.
FIG. 2 is an example of a waveform of no-load voltage that is the high-frequency voltage applied in the machining gap. A high-frequency voltage 201 applied in the machining gap has a waveform in which the voltage oscillates symmetrically around a ground (GND) electrical potential, which is a reference level. It is known that if the high-frequency voltage 201 of a frequency more than 1 megahertz is applied repeatedly and continuously, a finely machined surface can be obtained.
FIG. 3 is an example of a waveform of voltage when the high-frequency voltage is applied with a stop interval in which the voltage is not applied. FIG. 3 illustrates an example of a case when a high-frequency voltage 301 is applied repeatedly with a stop interval 302 of duration T2. A power supply source that generates this type of high-frequency voltage has been put in use. It is known that this type of high-frequency voltage improves the smoothness and finishing line of the surface of the workpiece.
It is known in the art to control the machining based on a real voltage (hereinafter, “inter-pole gap voltage”) in the machining gap in order to maintain a stable machining state. This control is carried out as follows. When the electrical discharge starts between the wire electrode and the workpiece, the inter-pole gap voltage decreases. The more the wire electrode approaches the workpiece and a discharge period becomes shorter, namely the more frequently the discharge occurs, the inter-pole gap voltage decreases further. Therefore, it is possible to estimate a width of the machining gap and determine whether the width is too short or too long.
Precisely, the inter-pole gap voltage is rectificated and transformed to a voltage that has one polarity. Based on this voltage, it is estimated whether a state in the inter-pole gap is an open state, a short circuit state, or a discharging state. The open state means that the electrical discharge does not take place. The short circuit state means that the wire electrode and the workpiece have short circuited. The discharging state means that the electrical discharge once occurred, but now the wire electrode and the workpiece have short circuited. A relative feed rate between the wire electrode and the workpiece is controlled (hereinafter, “an inter-pole gap servo”) based on the state determined in such a manner that the workpiece is machined in a stable manner.
The high-frequency voltage that has a frequency of more than several megahertz is beyond an operating limit of a rectification circuit of the wire electrical discharge machining apparatus. As a result, it is difficult to monitor the state in the machining gap and, therefore, it is difficult to control the relative feed rate between the wire electrode and the workpiece.
In other words, when a high-frequency power supply source is used, it is sometimes difficult to maintain the stable machining state. It is possible to maintain the stable machining condition when relative movement between the wire electrode and the workpiece is performed at a constant speed. An example of such machining is a machining when the machining amount changes very little, such as when performing finishing machining of a roughly machined workpiece. However, when the machining amount changes considerably as a result of change in the shape of the workpiece during machining, as it is difficult to maintain a stable machining state because it is difficult to monitor the state in the machining gap, traces of lines get easily formed on the machining surface. The shape of the workpiece may change due to distortion of the workpiece during the machining. As a result, at the time of a first-cut or if there is a variation in the machining amount, the high-frequency power supply source gives bad results.
Thus, although the wire electrical discharge machining apparatus with the high-frequency power supply source gives better results in general, there are drawbacks that need to be taken care of to meet strict market demands. One approach to solve this problem is to control the relative feed rate based on a state in the inter-pole gap. For example, Japanese Patent Application Laid-open No. H10-43951 (wire electric discharge machining device) discloses a technology to detect whether the wire electrode and the workpiece have made a physical contact and then control the feed rate based on a result of the detection in a manner to achieve stable machining condition. However, in this publication does not disclose a method to detect whether the wire electrode and the workpiece have made a physical contact.